Patients with dilated cardiomyopathy occasionally undergo coronary artery bypass or valvular surgery. Heart failure, characterized by discoordinate ventricular contraction and non-uniform electrical activation, may be present temporarily following a myocardial infarction or a surgical insult. At the conclusion of an operative procedure when other modalities have failed to improve cardiac function, temporary cardiac pacing can be employed to improve hemodynamics.
For temporary cardiac pacing, fine gage insulated cardiac pacing leads are commonly attached to the cardiac patient's heart. One type of lead placement frequently done in conjunction with open-heart surgery is transthoracic lead placement. Such placement is normally performed when the patient is on full cardiopulmonary support after cardiac surgery. With transthoracic lead placement, positive and negative cardiac pacing leads are usually placed on the right ventricular epicardium, often by means of a surgical needle. If unipolar (single-conductor) leads are placed, they will be spaced apart from one another in order to include ventricular tissue within the intended electrical circuit. In cases in which a bipolar (dual-conductor) coaxial pacing lead is selected for use in cardiac pacing, only one lead will be placed, since such a lead contains two discrete electrodes, spaced apart for adequate electrical performance. The cardiac pacing lead(s) are then threaded through the skin and are cut. Commonly the leads will then be tested for adequate electrical threshold.
Equipment used to conduct temporary cardiac pacing in conjunction with internally implanted cardiac pacing leads includes an external pulse generator, also known as a temporary pulse generator, and a pacing cable. Temporary pulse generators are electronic devices packaged in the typical box-like enclosures, are generally powered by mercury (alkaline) batteries, and when in use are typically pinned to the patient's gown so the unit will not be dislodged when the patient shifts position, stands or sits. Examples of temporary pulse generators which have been used by practitioners include the Medtronic Single-Chamber Model 5348 External Pulse Generator, and the Medtronic Dual-Chamber Model 5388 External Pulse Generator. Temporary pulse generators provide controlled electrical pulses to the heart of a pacing patient which stimulate the heart to beat in concert with the pulses provided. Accordingly, such pulse generators will provide paired negative and positive terminals, which are integrated within an externally accessible connector port.
A pacing cable is used to connect the pulse generator with the pacing leads extending from the heart and through the skin of the patient. The pacing cable will have a connector to mate with above-mentioned connector port of the temporary pulse generator, and when the connector and the connector port are mated thereby, a positive lead in the pacing cable is electrically connected to the positive terminal of the connector port, and a negative lead in the pacing cable is electrically connected to the negative terminal of the connector port. The conductors within the cable will typically be made of copper, surrounded by flexible material to provide electrical insulation, flexibility, and toughness. Typically the positive and negative leads within the pacing cable will pass through the cable without branching, and will simply terminate at the other end of the cable in terminals designed to accept and connect to the fine gage cardiac pacing leads. One type of connection used is a clip connection, commonly called an ‘alligator clip.’ Another type of connection commonly used involves a threaded cylinder-type terminal, in which the pacing lead is inserted into an aperture in the cylinder, and the threaded portion of the terminal is screwed downward, securing the pacing lead within the terminal. Once the wiring is in place, temporary pacing can begin.
Traditionally, when cardiac patients have required post-operative temporary cardiac pacing, right ventricular pacing has been performed, in which cardiac pacing leads including a positive electrode and a negative electrode are attached to the right ventricle of the heart. Recently, however, practitioners have found certain patients will benefit when cardiac pacing is performed in concert upon on the right and left ventricle, and have responded by reconfiguring temporary pacing apparatus to provide ‘biventricular pacing’, i.e., a pacing regimen in which both the right and left ventricles are subjected to electrical pulses intended to stimulate simultaneous contraction. In such instances, a left negative cardiac pacing lead is placed on the left ventricular epicardium, generally equidistant from the septum (LAD) when compared to the right negative cardiac pacing lead, and the negative cardiac pacing leads are usually combined at the point they attach to the pacing cable (i.e., the alligator clip or threaded cylindrical terminal). Positive leads are usually similarly combined when biventricular pacing is provided.
Other practitioners have also recently found that some patients, such as those who suffer from left bundle branch block, may benefit when left ventricular pacing, as distinguished from right ventricular or biventricular pacing, can be provided after surgery. However, the temporary pulse generators and pacing cables which are commercially available are designed to provide pulse generation for single-site (generally the right ventricle) ventricular pacing, and will provide biventricular pacing only when cardiac pacing wires are doubled up at the terminal interfaces of the pacing cable. As a result, a practitioner concerned with providing his patients the fastest post-operative recovery possible will find presently available pacing apparatus incapable of providing him the flexibility to select from among the alternative modes of ventricular pacing described above in a quick fashion commensurate with efficient clinical care.